The phenomenological theory of martensitic transformations is applied to the various observed transformation modes in the shape memory alloy nickel-titanium. Since the martensitic phase has a crystal structure which can be considered to be a monoclinic distortion of the orthorhombic B19 structure of gold-cadmium and is twinned internally, a treatment based on the formulation of Lieberman. Wechsler and Read has been used. New experimental evidence is presented to show that one of the observed twinning modes in the martensite can be identified as a type II mode, with the shear direction η1 being derived from a cell edge of the parent B2 phase. The theoretical predictions for this mode and the mode where the twinning is type I with K1 = (11 1 ̄)M are found to be in satisfactory agreement with the available experimental results. A third twinning mode, of Type I with K1 = (001)M is shown to be unable to produce the lattice invariant shear necessary in the martensitic transformation. © 1981.